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I'm a non-physicist with a basic high-school understanding of physics. I've always wondered what it is that makes things "solid". Why do molecules rebound from each other? There's just a bunch of tiny atoms there with (comparatively) large spaces between them. Why don't they just "slide" around or between each other?

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up vote 8 down vote accepted

What makes it solid is a combination of the uncertainty principle and Pauli's exclusion principle.

According to the uncertainty principle, electrons can't have a well-defined position if they have a sufficiently well-defined momentum (mass times velocity). For the energy of electrons to be low enough, the momentum also has to be low enough. That also means that the uncertainty of the momentum has to be low enough.

The uncertainty principle then implies that the uncertainty of the position of the electron has to be large enough, at least 0.1 nanometers or so - the atomic radius - for the kinetic energy to be smaller than a few electronvolts, a decent amount of energy that is comparable to the potential energy of electrons near protons if the electron clouds is similarly large.

Pauli's exclusion principle then guarantees that in each volume comparable to the volume of an atom, there can be at most 1 (or 2) electrons. That's why matter is impenetrable.

By the way, to derive the actual maximum density of electrons, one also needs to know the strength of the electrostatic attraction between the electrons and the protons that neutralize the charge. The Bohr radius goes like $1/\alpha$ - the inverse fine-structure constant - so if the electric force were stronger, matter could actually become denser.

This was an explanation why matter based on nuclei and electrons can't really significantly exceed the density of ordinary materials. Still, there are different phases. In gases, the molecules are separated by big gaps - so most of the space is empty and the exclusion principle is not too important. For liquids, the distance between the molecules is near the saturation point - like dense gases - but they still don't keep the shape.

Glass is an example of a liquid, in some sense, that is however behaving almost as solids. The canonical solids prefer to be crystals - like metals or diamond. In that case, it's energetically favored for the atoms or molecules to be organized to regular cubic or hexagonal or similar lattices. They like to keep this shape because it saves energy. It's still true that such solids are impenetrable because of the explanations at the top.

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Thank you! :) [Mandatory padding] – Vilx- May 2 '11 at 13:51
The idea that glass is really a liquid is controversial. – Keith Thompson Nov 14 '11 at 3:44
"in each volume comparable to the volume of an atom, there can be at most 1 (or 2) electrons" - one atom may contain dozens of electrons – M.M Mar 9 at 3:46
Dear @M.M - an atom may contain dozens of electrons but the volume of such an atom with a greater number of electrons is larger, too. The density of electrons per "volume of the hydrogen atom" is never too much higher. At the end, the density of elements is rather comparable. It's not an exact constant so there's a variability by an order of magnitude or so. But this is still a tiny variation relatively to the densities that are a priori possible - which may be smaller or greater by dozens of orders of magnitude, too. – Luboš Motl Mar 9 at 8:44

I was told in College that is was simple electrostatics. The electrons never come close enough to make the Pauli Exclusion, or uncertainty principle effective. It is simply that atoms repel each other at large distances due to the negative charge of the outer shell electrons.

Just wanted to throw that out there.

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Does this really make sense? Weren't atoms supposed to be neutral, especially over large distances? And wouldn't this then prevent molecules from forming? – Vilx- May 3 '11 at 7:30

In the begining the earth was (void) and without form.... God said let there be light. In the void a boundry layer was formed in the infinite poteintials of the large and small. First the Plank length for the small and the black hole for the large. this is charged by vibrations between the negative and positive energy (=0). This formed the first matter by light being caught in a (unstable) feedback loop wich defines time. Matter ends up being solid because of relativity. Time for the particle ends up different that ours and the apperance of solids and fliuds and gases form from the interation between time/light to form the stucture. This would suggest that time is one with element and gravity is the radiation from the derivation in time.

This makes more sense than a big bang, its actually a big crunch......

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.... This is poetry, right? – Vilx- Nov 14 '11 at 8:41
Or theology, perhaps, can't tell those apart. But certainly not science. – MSalters Nov 14 '11 at 12:44

protected by Qmechanic Oct 22 '13 at 21:00

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